A wireless position detection system for terminals using wireless LAN previously applied for by the present inventors is disclosed in U.S. Pat. No. 101,463,608. In this system, the wireless signals exchanged between the terminal and the base station are received by multiple other base stations separate from that base station, and the position of the terminal is detected based on the reception timings for the respective measured signals. A chip clock differential of ±25 ppm is specified for base stations conforming to IEEE802.11. This clock contains an individual (error) so as shown in FIG. 7, the clock differential between base stations varies with time. However, if within about a dozen seconds, this change is nearly the same as in the linear equation. Therefore, as shown in U.S. Pat. No. 101,463,608 previously disclosed by the present applicants, multiple wireless signals are required from the base station in order to calculate the clock differential between base stations.
On the other hand, when operating these type of wireless position detection systems in environments with walls that reflect the wireless signal such as within buildings, the waveform of the wireless signal becomes distorted by the multipath environment, and the error when determining the reception timing at each station increases. In the method disclosed previously in JP-B No. 030281/2003 by the applicants, the base station receives multiple signals from the terminals and reduces the ranging error on the multipath environment by averaging the reception timing for each measured signal.
In view of the above two points, utilizing multiple wireless signal proves effective. However, the time required for wireless communication increases with the number of signals. In particular, when detecting the positions of high speed mobile units, every effort must be made to avoid longer wireless communication times during the sending and receiving of multiple wireless signals.
[Patent document 1]
JP-A No. 014152/2002